This invention relates to heat exchangers in general, and specifically to an improved method of forming a fabricated heat exchanger tube.
Cross flow heat exchangers, of a type used in automotive applications and elsewhere, include a regularly spaced series of generally straight, flat and parallel tubes, through which a first fluid (such as refrigerant or engine coolant) flows in one direction, and over which a second fluid (typically ambient air) flows in another direction. Thin, corrugated air fins or air centers are brazed between the flat, parallel tube outer surfaces. In high pressure heat exchangers, such as condensers, a solid, extruded tube is often preferred, because of the inherent ability of integrally extruded structural webs to support the internal tube pressure. In lower pressure heat exchangers, such as engine cooling radiators, internal webs are not as necessary, and fabricated or folded tubes made from aluminum strip are typical. Sheet metal strip has the advantage over solid extruded material of being easier to coat with a clad layer of braze material, which melts and provides the raw material needed to create solid joints at the various component interfaces in the heat exchanger core. Even though internal pressure support is not as necessary with a low pressure tube, central structural support can be beneficial if the tube is widened, since widening makes the tube more vulnerable to outward bulging, even with fairly low pressure inside the tube. Some radiator designs use double tube rows in order to achieve sufficient coolant fluid capacity, and it would be beneficial to replace the double row with a double wide single tube.
In order to achieve sufficient structural strength in a wide fabricated tube, several basic prior art designs have been used, all of which are variations, sometimes very minor variations, on the same basic theme. The basic objective is to fold the strip over and in on itself in some fashion to provide an internal strengthening wall. Typically, the internal strengthening wall is a central (or nearly central) abutment of two flanges, welded or brazed together. One variation folds the sheet down the middle, with each section folded over into a right angle flange and then folded again toward the other until the flanges abut at the center to create a xe2x80x9cBxe2x80x9d shaped cross section. The abutting flanges may be welded or brazed together by any desired joining method. An example may be seen in UK Patent Specification 1,149,923. Another basic variation folds the sheet down the middle, but with each section folding in opposite directions so that the flanges each abut to opposite sides of a central spine, creating a xe2x80x9cZxe2x80x9d shaped cross section. An example may be seen in U.S. Pat. No. 4,633,056, where the edges of each section abut to the central spine either at a sharp edge, or with a bent over, curved edge. A variation of the xe2x80x9cZxe2x80x9d shape, seen in U.S. Pat. No. 2,655,181, bends each edge into an L shaped foot and abuts one L shaped foot to either side of a central spine, creating a very strong, three layered central wall. The same L shape can also obviously be used for the abutting flanges in the xe2x80x9cBxe2x80x9d shaped tube, providing for more contact area for brazing or welding, although requiring more sheet stock, at extra cost, for the same size tube. An example of this variation may be seen in U.S. Pat. No. 6,000,641, a patent which also recognizes an additional problem with a xe2x80x9cBxe2x80x9d cross section tube, which is the effect of the central seam on the brazing process, discussed in more detail below. With any tube cross section, the standard folding process in use today is a continuous series of rollers that progressively folds and forms the tube to shape, as a length of strip is fed through the rollers.
While the basic shape and cross section of essentially every possible variant of a folded, fabricated tube has been suggested or disclosed, the manufacturing methods and processes still have room for improvement, especially in the area of the welding and brazing of the seams. As part oF the brazing process, it is standard practice to apply a flux layer over the clad layer. Flux may be applied by electrostatic powder adhesion, or by slurry spray, to the interior surface of the strip, but either technique wastes flux by applying it to more surface area than just the contacting interfaces. If flux is applied after the tube is folded, it is difficult to get flux into the seam between the abutted flanges, without dipping, flooding or injection techniques that also waste flux.
One proposal to selectively apply flux to a xe2x80x9cBxe2x80x9d tube may be seen in published European Patent Application EP 0 982 095 A1. As disclosed there, a standard flux composition is applied continuously and indirectly to the central inner surface of a progressively forming tube, in a stripe where it will contact the edges of the abutted central flanges as they are folded down. This indirect flux application is done with a roller, to which the flux paste is continuously applied, and which in turn rolls along the tube central inner surface of the strip to leave behind a thin layer of flux. The rollers that seat the flange edges down into the flux stripe are located downstream of the flux roller. While this is a continuously acting and less wasteful application of flux, it is limited in that only a thin layer of flux may be applied, and of a fluid consistency suitable to the spreading action of the roller. An excessively viscous paste will not be fluid enough to be applicable by a roller, nor would a roller be inherently capable of applying a thick, viscous layer. While such a thin layer is well presented to the terminal edges of the abutting central flanges where they engage the inner surface of the tube, it is not as effective in reaching the much wider interface between abutting flanges themselves, which forms the central seam of the tube.
As noted above, there is another potential problem recognized in the art with the standard xe2x80x9cBxe2x80x9d shape tube cross section, a problem inherent in the shape and consequent effect of the central tube seam during the brazing process. U.S. Pat. No. 6,000,461 recognizes that the central seam creates a curved, converging depression in the outer surface of the tube, which can, through strong capillary action, draw or scavenge melted braze cladding away from the surface of the slotted header plates into which the tube ends are typically inserted. This jeopardizes the strength and integrity of the braze joints at the header slots. One solution is that extra thick braze cladding material could be provided on the outside of the tube, the header, or even to the fin material, but this would be expensive and not desirable. The patent noted proposes to skive out and enlarge the seam into a wide, non convergent gap, which would, so it is claimed, reduce the capillary action. Regardless of its effect on capillary action, such a widened seam would weaken the tube and subject it to debris trapping and corrosion. An extra manufacturing step such as skiving also adds cost.
Another patent, U.S. Pat. No. 6,129,147, seeks to control the shape of the seam by making an extra fold of the edge of the strip stock up between the pair of abutting flanges, which divides the gap of the seam in two, in effect. This also creates a very impractical and oddly shaped tube cross section, and tooling to actually create such an odd fold would be difficult to devise. Even so, dividing one deep seam into two seams would not solve the braze scavenging, seam capillary problem totally, since each side of the divided seam could still create some capillary action.
The subject invention provides an improved method for producing a folded tube of the type described above. A thick bead of flux and/or braze paste mixture is directly deposited on the inner surface of the strip as it is progressively folded to shape. As the integral flanges are bent inwardly and into abutment with each other, their lower edges are forced downwardly into the bead, and a layer of the thick bead material is squeezed up into the seam interface between the abutting flanges. This provides an excellent bond, with no additional steps necessary beyond the depositing of the bead material itself. The viscosity and consistency of the bead does not have to be limited or tailored to a rolling on process. Extra braze alloy can be provided in the bead to strengthen the bond and limit the scavenging action of the seam noted above, without any deleterious change to the tube cross section or any complication of its folding process.